Judging by the movie industry's current output, adolescence and the 1970s are the most misunderstood times of all. In her debut feature, "Slums of Beverly Hills," director Tamara Jenkins reverses both trends, capturing the heartbreak of growing up in an era whose veneer of freedom set its children adrift on a sea of painful choices and elusive dreams.

With unclouded self-awareness, Jenkins reimagines her self as Vivian Abramowitz (Natasha Lyonne), an awkward almost-woman struggling to reconcile the uncertainty of her family's nomadic existence with the images of affluence just beyond their reach in the California of 1976. A child of divorce, she's forever being awakened in the dead of night by her perennially broke father, Murray (Alan Arkin), to vacate another seedy, yet unaffordable, set of living quarters on the wrong side of the Hollywood Hills. "That place was a shithole," he repeatedly dismisses, masking his shame at being unable to provide a better life for Vivian and her two brothers. The destitute Abramowitzes, it's clear, will never have reason to confuse themselves with the prosperous, happy clan on "Eight is Enough."

If Vivian's daily existence is in a constant state of flux, her body's changes have attained warp speed. "She's stacked," notes her amazed brother of the pubescent Vivian's exploding bustline, not sure if he's surveying flowering beauty or a rapidly unfolding freak show. Vivian isn't sure, either. From all that she sees, sexuality is more trouble than it's worth. Her new voluptuousness earns her the attentions of a neighbor (Kevin Corrigan) whose advances are as frightening as they are thrilling. In addition, she witnesses firsthand the emotional wreckage that the need for companionship wreaks on her beaten father and her lunatic cousin Rita (Marisa Tomei), a drugged-out, newly pregnant drifter whose arrival only adds to the family's troubles. If this is the aftermath of the sexual revolution, where are all the winners?

One woman's sob story is another's ribald hilarity, and Jenkins finds room for both in her funny, affecting script. She's ably abetted by the presence of the ever-insightful Arkin, who instinctively hones in on the comedy and the tragedy of his pivotal character. Tomei's performance is the film's one stumbling block: Consistently too broad for such nuanced material, she appears unable to shake the need to project she picked up performing Shakespearean comedy in New York's Central Park. She's so far out of step with the rest of the cast that she appears to be in another movie entirely.

The real discovery is Lyonne, whose obvious talent gives firm anchor to even the most unpredictable of the plot's twists. Simultaneously beautiful and gangly, she's the finest doppelgänger Jenkins could want. If she's paid close attention to this film's message of hard-won identity, Lyonne will be well-equipped to handle the demands of a business that tends to treat individuality like a weed in need of pulling. And when less sympathetic directors begin to turn her down for parts for looking "too something" -- too Jewish, too frizzy-haired, too unconventional -- she'll already have learned that defining yourself by someone else's limited vision is just slumming.